A top US official has defended as legal and ethical its use of drone strikes to target terrorism suspects.

The officials comments on Monday by White House counter-terrorism adviser John Brennan was the most direct acknowledgement yet of the clandestine programme.

"In full accordance with the law ... the US government conducts targeted strikes against specific al-Qaeda terrorists, sometimes using remotely piloted aircraft"

- John Brennan, White House counter terrorism adviser

"Yes, in full accordance with the law - and in order to prevent terrorist attacks on the United States and to save American lives - the United States government conducts targeted strikes against specific al-Qaeda terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones," John Brennan said in a speech at the Woodrow Wilson Centre in Washington.

Brennan said President Barack Obama had asked the administration to be more open about the drone programme.

He defended the attacks as legal and noted broad discussion about the use of the strikes.

'Not legal analysis'

"We discuss. We debate. We disagree," he said. "We consider the advantages and disadvantages of taking action. We also carefully consider the costs of inaction and whether a decision not to carry out a strike could allow a terrorist attack to proceed and potentially kill scores of innocents."

Brennan's remarks were interrupted when a member of the anti-war group, Code Pink, criticised the use ofdrone strikes. The female protester was removed from the room by a police officer.

The American Civil Liberties Union, a longstanding critic of the tactic, welcomed the acknowledgement of the programme, but said Brennan did not provide sufficient legal justification.

Brennan said some of the documents, captured by the US Navy Seal from Bin Laden's compound due to be published online this week, showed the late al-Qaeda leader urged subordinates to flee for places "away from aircraft photography and bombardment."

The material is due to be published online this week by the Combating Terrorism Center at the US Military Academy at West Point.

A suspected US drone strike most recently left four dead in Pakistan, and the issue has proved a major hurdle in relations with Islamabad."We continue to believe that the administration should release the Justice Department memos underlying the programme - particularly the memo that authorises the extra judicial killing of American terrorism suspects."

Following an attack that left two dozen Pakistani soldiers dead last year, Islamabad has demanded a halt to the drone programme before reopening NATO supply routes through its territory.

Earlier this year, Obama was asked about the drone programme during a virtual town hall meeting and stressed that the US was "judicious" in its use of the technology.

Attorney General Eric Holder has offered a legal defence of the killing, last year in Yemen, of al-Qaeda cleric Anwar al-Awlaki, a US citizen.